Emulsion polymerization can be used to produce polymer dispersions in water, more commonly referred to as latexes. Polymer latexes are used in many applications such as adhesives, carpet backing, caulks, coatings, elastomers, paints and packaging. Latexes are typically stable colloidal suspensions of polymer particles in water. When the particles are sufficiently small they can be individually stabilized through the use of surfactants and kept dispersed through Brownian motion.
The emulsion polymerization process can be performed in an aqueous medium in the presence of a surfactant or combination of surfactants. Surfactants play a role in both the formation of emulsion polymer latexes and in stabilizing the final polymer dispersion. Water soluble free radical initiators such as ammonium, sodium, or potassium persulfate can be used to rapidly produce high molecular weight polymers at relatively high solids content and low viscosity. The process requires the emulsification of the monomers in the aqueous phase through the use of surfactants. Other ingredients used in the emulsion polymerization process may include buffers and chain transfer agents and minor amounts of polymerizable acids such as acrylic acid, methacrylic acid, or itaconic acid. Since the surfactant cannot be easily removed from the final latex, normally remaining imbedded in the polymer matrix, the quantity and type of surfactant used can determine many of the characteristics of the latex and performance characteristics in a final application.
The surfactants remaining in a latex can have deleterious effects in the final application. For example, water sensitivity and poor adhesion can be caused by the migration of surfactants within a dried polymer matrix. Surfactants can migrate to form domains within the polymer matrix as well as migrate to surfaces and interfaces. Surfactant domains at the surface can lead to the formation of micro-voids that can leave the polymer film vulnerable to the penetration of moisture leading to water sensitivity and degradation of polymer film strength. The presence of surfactant at the polymer-substrate interface can lead to reductions in adhesive strength.
Attempts to minimize the adverse effects of surfactants in emulsion polymer applications have resulted in surfactants that become incorporated within a latex through covalent bonds, or through a combination of covalent and ionic interactions. Such “polymerizable surfactants” include surfmers and non-migrating surfactants. However, the use of polymerizable surfactants can present difficulties in the polymerization process such as their tendency to form coagulum. Therefore, a need remains for alternative systems that mitigate the problems associated with surfactants mobility while providing increased water resistance and improvements in properties such as block resistance, adhesion, optical clarity, leveling and freeze-thaw.